Most traditional ceramic manufactured products, such as tiles and sanitary ware, are made of a ceramic body that confers shape and mechanical properties to the object; the ceramic body generally has some porosity and poor aesthetic qualities.
Said ceramic body, which is defined “green” or, alternatively, “fired”, is usually coated with a ceramic layer, called ceramic glaze; the ceramic glaze is sintered by firing, in such a way to gain suitable superficial aesthetic qualities and, in the meantime, to become a fluid-proof barrier; as a matter of fact, after firing, the ceramic glaze has usually no porosity and is generally resistant to abrasion and to the attack of chemical agents.
Glaze is mainly applied on the surface of the ceramic body dispersed/suspended in a opportune vehicle, usually water, or, in some special application, by dry dusting a dry mixture over the surface of the ceramic body. Traditional liquid ceramic glazes are suspensions of various powdered minerals and metal oxides that can be applied by directly dipping pieces into the glaze, pouring the glaze over the piece, spraying it onto the piece with an airbrush or similar tool, with a brush, or with any tool that will achieve the desired effect.
Liquid ceramic glazes, also called ceramic glaze slips, generally contain, finely dispersed in water, silica to form glass, also in form of frit (pre-fired vitreous component); mixtures of metal oxides, usually in the form of pre-treated natural occurring minerals, such as alkaline earth metal oxides which act as a flux and allow the glaze to melt at a particular temperature; alumina to stiffen the glaze and prevent it from running off the piece; ceramic pigments, such as oxides or carbonates of transition metals.
Since most of the ingredients cited above are heavy ingredients and in order to obtain a proper coating before and after firing, it is necessary to add some particular additive into the liquid ceramic glazes. These additives, often organic in nature, are added, singularly or as compositions, to glazes to give them specific properties that are required during application. They do not participate directly to the vitrification process, but are able to give special characteristics to the glaze slip useful for the subsequent application on the ceramic body or to the formed glass for successive treatments.
These ceramic glaze additives are well known in the art and further information can be found in literature, for example in Fortuna D., “Sanitaryware”, Gruppo Editoriale Faenza Editrice, p. 61-64 (2000) and Stefanov S. and Batscharow S., “Ceramic Glazes”, Bauverlag GmbH (1989).
The most common additives for ceramic glazes are: suspending agents, such as water-swellable clays; thickening agents, such as carboxymethyl cellulose, alginates, natural gums and acrylic (co)polymers; preservatives, biocides, antifoams, dispersants (fluidizers), such as medium/low molecular weight acrylic acid (co)polymer; binders; deflocculants; levelling agents and plasticizers.
Many of these additives are added into the glaze slips as powders. Powders by their nature have very large surface areas susceptible to humidity and/or bacterial growth.
Handling of such powders and dust generation during processing create environmental and health problems that must be dealt with by the manufacturer and the customer.
In addition suspending agents and thickening agents in form of powders are difficult to dissolve in the thick glaze slip and, if not stirred for enough time and/or with a high shear mixer, they can create lumps or aggregates in the glaze slip. After preparation, the glaze slips are sieved in order to eliminate residual impurities and aggregates. If not completely dissolved, the lumps or aggregates of the additives can increase considerably the time required for the sieving. Moreover, a partial dissolution of the rheology modifier can require a time-consuming correction of the viscosity of the glaze slip or, if not corrected, can cause serious glazing defects on the final products, such as leveling problems, running or crawling, which are well known to those expert in the art.
Furthermore the exact dosing and in-loading of the powdery additives, which have usually different densities and different particle sizes, are source of further difficulties.
A typical solution to these problems commonly used in many fields is to granulate the powdery compounds or compositions. Unfortunately the granules obtained during the granulation process are different in their forms and dimensions, thus making it necessary to sieve the granulated material, for the purpose of selecting the granules presenting dimensions above a minimum value. Moreover granulation does not eliminate dust. In fact, a percentage of this dust, even if small, remains imprisoned in between the granules and tends to spread itself.
It has now been found that the use of compositions of these additives in form of extruded pellets can solve all the above mentioned difficulties. The composition and dimensions of the pellets can be easily controlled in order to avoid hazard and to optimize processing, handling/shipping, in-loading dosing, etc. At the same time the extruded pellets are really compact and do not produce dust when handled and have a low dissolution rate, compared to the powders, which reduce significantly the formation of lumps in the glaze slip.
As far as the Applicant knows, the use of extruded pellets obtained by extrusion of a mixture of two or more ceramic glaze additives for the preparation of glaze slips have not been described in the literature.
By “pellet”, we mean any solid shaped composition, including but not limited to, tablets, pearls, flakes, briquettes, bars, or blocks.